Preheat oven to 400º. Place hero rolls on half sheet pan and set aside.

In a large mixing bowl, combine celery, carrots, onions, garlic, parsley, bread crumbs, 1/4 cup Parmesan, salt, pepper, egg, and ketchup. Mix together until well combined with a large spoon. Add beef and pork, mixing together by hand until just combined (don't over mix to maintain tender meatballs). Form 1 1/2" meatballs and place in an oven-proof dish or sheet pan. Do not overcrowd. Bake until meatballs are browned, 20 minutes.

Meanwhile, toast rolls and warm marinara in a small sauce pan. Spoon marinara over meatballs and top with mozzarella. Heat in oven until cheese is melted.

Place meatballs in toasted hero rolls and top with remaining Parmesan. Serve immediately.

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